Pages

Sunday, July 7, 2013

Uranus or Bust (and on a budget)

Given my interest in
future planetary missions, I regularly look through lists of missions submitted
to space agency mission selection competitions.
I also read through the abstracts of mission concepts presented at the many
planetary science and engineering conferences each year. Uranus is trending.

Why the interest now? First, the 2011 Decadal Survey ranked a $2B
Uranus orbiter and probe mission as a priority to launch in the coming
decade. (Alas, new budget realities make
any such mission look 20 years away or more now.) Second, the Uranus-sized worlds are proving
to be common in other solar systems and may be the most common type of planet
in the galaxy. Our only up close
examinations of planets in this class were the flybys of Uranus and Neptune in
the 1980s by the Voyager 2 spacecraft that carried 1970s vintage
instruments. Third, NASA’s development
of the light and relatively cheap ASRG plutonium-based power systems enables cheaper
missions than were possible with the older, heavier power systems. And fourth, the changing outer planet
alignments have made gravity assists from Jupiter and Saturn to shorten flight
times to Neptune impossible the current mission planning window. Jupiter is still available for Uranus
missions in the coming decade.

Voyager 2’s cameras saw a
near featureless cloud deck at Uranus.
More recent images from observatories looking in different portions of
the spectrum have shown that Uranus has an atmospheric circulation system as
complex as Jupiter’s or Saturn’s. From a
presentation on a Uranus atmospheric probe mission by Mark Marley and
colleagues at NASA’s Ames Research Center.

A presentation by Mark Hofstadter of JPL and member of a Uranus science
working group, has laid out the scientific case for a mission to this world and
its prospects. (A European vision of a
similar scale Uranus mission can be read
here.)

For Uranus, science
objectives break into several broad classes of studies. Scientists want to send a probe into the
atmosphere for detailed composition measurements of the atmosphere to better
understand how Uranus formed and how its structure has evolved. Remote observations of Uranus to study its
weather, understand its heat balance, and probe its interior are another
priority. Researchers would like to study
the planet’s ring system many and many moons up close. And finally, they would like to re-measure Uranus’
highly unusual magnetosphere which is unlike any other planet’s except for Neptune’s.

The ideal mission
architecture would combine an atmospheric probe with an orbiter, as proposed by
the Decadal Survey. (You can find a copy
of the mission study at this website.) The atmospheric probe would gather
compositional information that is not available any other way. An orbiter would stay within the Uranus
system for prolonged observations of the planet, rings, and moons. From orbit, the spacecraft would also
traverse many locations within the magnetosphere allowing a study of its
structure not possible from a flyby spacecraft that travels a single path.

The scientific case for
exploring Uranus is solid and gaining attention. NASA’s shrinking planetary science budget,
however, can no longer support the approximately $1.8B orbiter and probe
recommended in the Decadal Survey.
Mission architects, however, have begun to look at alternative, cheaper
missions that could fit in the New Frontiers (~$1B) and Discovery (~$500M)
mission programs. These cheaper missions
would reduce costs by conducting only a portion of the Uranus science goals.

The team examining
Uranus science goals for the Decadal Survey divided the science goals into Tier
1 (highest priority), Tier 2 (high priority), and Tier 3 (highly
desirable). While the Decadal Survey
endorsed a mission that would address goals from all three tiers, a New
Frontiers mission would likely focus on only the highest priorities.

Estimated
costs of Uranus mission elements from the DecadalSurvey’s Ice Giants Decadal Study Mission Concept Study. The Tier 1 (highest priority) science orbiter
costs are near the cost of a New Frontiers mission. Many observers have commented that the Decadal
Survey mission costs were conservative (i.e., erred on the high side) so a New
Frontiers Uranus orbiter might fulfill all the Tier 1 goals.

The most straightforward
way to enable a cheaper Uranus mission would be to do either an orbiter or an atmospheric
probe mission. The Decadal Survey’s minimum
orbiter mission cost (~$1.2B) was close to the close to the expected $1B cost
cap of future New Frontiers missions.
The Decadal Survey report on the conceptual design of a Uranus
orbiter-probe mission listed two Tier 1 goals:

“Determine the atmospheric zonal winds, composition,
and structure at high spatial resolution, as well as the temporal evolution of
atmospheric dynamics.” Instruments: wide angle camera with multiple
filters and a visible/near-IR (Vis/NIR) mapping spectrometer

“Understand the basic
structure of the planet’s magnetosphere as well as the high-order structure and
temporal evolution of the planet's interior dynamo.”

An
orbiter would allow many regions of the Uranus system to be explored. This figure is from the mission concept study
and shows the path of the orbiter in a Uranus-fixed coordinate system.

If the spacecraft’s orbit were
designed to enable close flybys of Uranus’ larger moons, these three
instruments would also address two Tier Two science priority objectives: “Remote
sensing observations of large satellites,” and, “detect induced magnetic
fields that would be indicative of interior oceans [in Uranus’ moons].” However, the best orbits for meeting the
Uranus observations are not the same as those for close flybys of the
moons. Accomplishing both would require
a longer and more expensive orbital tour (~$26M more) that might not fit within
a New Frontiers budget.

The three Tier 1 science
instruments are based on mature technologies, and similar instruments have
flown on numerous missions. They could
have a combined mass less than 15 kg. By
comparison, just the Cassini Saturn orbiter’s camera system is almost 58 kg and
is just one of twelve instruments. A
Uranus mission that focused just on Tier 1 science would indeed be a tightly focused
science mission. (If money for a the
full $1.8B Flagship mission were to become available, the mission would add
fields and particle sensors, a mid-infra-red thermal detector to measure the
distribution of thermal emissions, a narrow-angle camera, and a UV imaging
spectrograph, and a nephelometer to detect clouds to the atmospheric probe.)

To my knowledge, no
formal analysis of a New Frontiers class Uranus atmospheric probe mission has
been performed. However, the
requirements and complexity of such a mission would seem to be similar to that
of a Saturn atmospheric probe mission.
That mission has been studied and been judged to fit within a New
Frontiers budget. (A Uranus mission
would have a longer flight time that would raise costs somewhat compared to a
Saturn mission.) Requirements for the atmospheric probe portion of a Uranus
orbiter also were examined by as part of the Decadal Survey and continue to be
studied at NASA’s Ames Research Center.

The Decadal Survey
analysis ranked the goals for an atmospheric probe mission lower than those for
the Tier 1 orbiter science. A single
Tier 2 goal was identified for the atmospheric probe:

“Determine the noble gas abundances
(He, Ne, Ar, Kr, and Xe) and isotopic ratios of H, C, N, and O in the planet’s
atmosphere and the atmospheric structure at the probe descent location.” Instruments: Mass spectrometer and pressure, temperature, and
acceleration/deceleration sensors.

Further
analysis by the NASA Ames team suggests that the atmospheric probe should be
able to survive to below the bottom of the methane clouds where the atmospheric
pressure would be five times that at sea level on Earth. (Measurements from below the lower water clouds
– a key goal for the Galileo atmospheric probe at Jupiter – also would be
beneficial, but these are too deep and the pressure too great at Uranus to
achieve within any mission currently conceived.) Uranus is believed to contain 30 to 50 times
more carbon (a key element of methane) than would have been the mean in the
solar nebula from which the planets formed.
A key question for planetary science has been how the enrichment of
carbon and other elements occurred.
Measurements of the ratios of key elements and their isotopes would help
determine whether Uranus-class planets are cores of planets that failed to grow
into Jupiters or super-Jupiters or represent an entirely different path of
planetary creation.

The
proposed Saturn atmospheric probe would be carried to its destination by a
simple carrier craft that would also act as a data relay that would collect and
return the atmospheric probe’s data to Earth.
As envisioned by the Decadal Survey, the carrier craft would not carry
any instruments of its own. A Uranus atmospheric
probe carrier craft also probably would not carry any instruments.

If
the scientific community continues to support the same priorities as the team
that examined Uranus mission goals for the Decadal Survey, the orbiter would
address Tier 1 goals while the atmospheric probe mission would address Tier 2
goals. That would give the nod to the
orbiter mission.

There
is one programmatic hitch to using the New Frontiers program for either a
minimalistic orbiter or an atmospheric probe mission. New Frontiers missions are selected from a
list of candidate missions pre-selected by the Decadal Survey. The current list includes missions to Venus,
the moon, a comet, the Trojan asteroids, Io, and Saturn, but not Uranus. However, NASA can change the list and has
suggested (see this presentation)
that it would be open to a request by the planetary science community to change
the list in light of the smaller budgets that have become reality since the
Decadal Survey. There will also be a
science community review of the Decadal Survey in the second half of this decade
that will review the candidate list and could revise it. Either approach would add a Uranus mission to
the list of candidates, but Uranus would be in competition with five to six
other exciting destinations.

The new interest in
Uranus missions has also led mission architects to look at even lower cost
Uranus missions than those described above.
In addition to the New Frontiers program, NASA has Discovery missions
that cost approximately half that of New Frontiers missions at <$500M. To date, none of the twelve selected missions
will have flown beyond the asteroid belt.
Flying to any outer solar system destination poses considerable
challenges within the Discovery program cost cap. The long flight times require greater spacecraft
reliability and require funding many years of flight operation costs. Missions beyond Jupiter (and some within the
Jovian system) also require plutonium power supplies, although NASA in the past
has offered to help defray the costs of using an ASRG system.

To fit within the
Discovery cost cap, a Uranus mission would be limited to a simple flyby and likely
would carry only one or two instruments.
The spacecraft also would need to be able to conduct science beyond what
Voyager did in 1986, a tough task with just one or two instruments. Recently, though, a proposed Uranus Discovery
mission has been studied that would do that with just a single instrument.

To understand the
proposed mission, let me give a bit of background. Gaseous worlds have internal turbulence that
generates acoustic wave oscillations that propagate to their surface. (The closest equivalents in a solid rocky
world are earthquakes that periodically produce movement at the surface.) Solar astronomers have exploited this
phenomenon through helioseismology to study the interior “seismology” of the
sun. The giant planets, including
Uranus, can also be studied using the same technique. (Uranus is referred to as an ice giant
because the bulk of its composition (such as water) would freeze if exposed to
space that far from the sun. However,
the pressure and heat within Uranus keeps the deep interior fluid except for a
rocky core.) A Doppler spectrographic imaging instrument could measure the
oscillations and reveal the interior structures of outer planets. To date, however, no spacecraft has carried
one to an outer planet.

Acoustic
wave seismology for giant planets including Uranus. From a presentation by Steve Matousek and colleagues at
JPL on the Uranus Explorer concept.

A JPL team has proposed
a Discovery-class mission that would carry a small, 35 cm telescope that would
house both a Doppler imager and a visible camera. The spacecraft would first flyby Jupiter to
study its interior. Approximately seven
years later, the spacecraft would repeat those measurements at Uranus.

The Uranus Explorers
Doppler imager would fulfill two goals for Uranus:

Acoustic waves have been
detected on Jupiter, but not so far for the other giant planets. It may be that their greater distance puts the
detection of these waves below the measuring threshold for Earth-based
telescopes. For Uranus, these waves may
be especially weak. Acoustic waves
are created by internal motions produced by heat flow within the planet. Key trace gases such as carbon monoxide in
the upper atmosphere demonstrates that this convection occurs on planets such
as Jupiter. (Carbon monoxide forms in
the deep, hot interiors but chemical reactions in the cooler upper atmosphere
eventually converts it to methane. For
carbon monoxide to be detectable, it must be regularly replenished by
convection.) Uranus, however, is unique
among the giant planets in having a low heat flow and a lack of these key trace
gases. If the Doppler measurements fail
to measure acoustic waves at Uranus, this could put a threshold on the amount
of internal convection within the planet.

The single, quick transit of the Uranus system would not
provide time to meet all the science goals for studying the moons and rings. However, during the hours around closest
approach, the visible camera would address two other goals:

Determine
the geology, geophysics, surface composition, and interior structure of large
satellites.

Determine
the composition and dynamical stability of the rings and small satellites.

In researching this
post, I have come to realize how diverse Uranus’ moons are. Like Saturn, Uranus has many small moons in
and near the ring system and a diverse group of medium-sized moons further out
(see this presentation).
(Uranus, of course, lacks a truly large moon equivalent to Saturn,
though.) Uranus’ Ariel, like Saturn’s
Dione, shows signs of internal activity that have partially resurfaced its
surface. Two of Uranus’ moons are large
enough that astronomers suspect that they may harbor interior oceans.

The JPL presentation
focuses on demonstrating that a scientifically compelling Discovery-class
Uranus mission is possible. With further
study, I suspect that a proposal team building on this start would find
additional, fairly low cost ways to enhance the science. Careful design of the flyby trajectory could
have the spacecraft travel behind the rings and allow the transit of the radio
signal from the spacecraft to measure the structure of the ring system.

If an ultra-stable
oscillator was including in the communications system, the radio transmissions
could be used to measure gravity fields.
Improving on Voyager’s gravity measurements of Uranus itself would be
difficult because the spacecraft would have to fly dangerously close to the top
of the atmosphere where inner ring particles could present a hazard. However, the craft could be sent past one of
the moons to use gravity to measure its interior.

International
partnerships might allow adding a simple instrument or two like this to the
craft with little cost to NASA. One high
priority instrument might be a visible/near-infrared mapping spectrometer that
could measure trace gases in Uranus’ atmosphere. Measurements of trace gases (or their
absence) would provide a second way to estimate the internal turbulence of
Uranus. Alternatively, if the flyby
spacecraft carried a magnetometer and its trajectory carried it close to one of
the larger moons, it could look for changes in the magnetosphere that could
confirm an ocean. A magnetometer also would
provide a second look at Uranus’ magnetosphere.

The JPL presentation describing
the proposal calls the mission simply the “Uranus Explorer.” Because the spacecraft studies two very
different giant planets, I think a better name might be something like,
“Echoes: Giant Planet Seismology.” JPL’s
Uranus Explorer Discovery mission concept is in the early stages of study. What I find exciting is that the results so
far suggest that a compelling mission to Jupiter and Uranus could be possible
within the tight cost constraints of the Discovery program.

Editorial Thoughts:
Before I did the research for this post, I had almost given up hope for a
return to Uranus within my lifetime.
(I’m in my mid-fifties.) Now it
appears that a mission could fit within either the New Frontiers or the
Discovery program, and I’m hopeful. However,
I also recognize that NASA’s shrinking planetary science budget means fewer
missions will be flown. Competition from
other exciting solar system destinations will be fierce. But I am hopeful.

As I wrote this post, I
found myself wondering which of the mission options I personally find most
appealing. I concluded that I believe
the combination of a flyby spacecraft with the Doppler imager-camera with an
atmospheric probe would provide the combination of science that I believe is
most compelling. I don’t know if this
could be done within the cost cap of a New Frontiers mission or not, and
recognize that the scientific community may well decide that another mission
configuration provides better science.
For me, though, the combination of examining the interior of two planets,
measuring the composition of a new class of worlds with a probe, and imaging
the other half of Uranus’ moons is compelling.
But I would be excited by any mission to Uranus.

5 comments:

This is one of the best posts I have ever read about a mission to planet Uranus. In my opinion Uranus and Neptune are almost forgotten since Voyager 2... Only Hubble and Keck have a glipse at these planets from time to time. It is a pity, considering especially the interesting geology of their Moons (particularly Miranda and Triton) and the complex atmospheres of these two giant planets.

Keep this blog live. It will be very interesting to read future posts.

This is one of the best posts I have ever read about a mission to planet Uranus. In my opinion Uranus and Neptune are almost forgotten since Voyager 2... Only Hubble and Keck have a glipse at these planets from time to time. It is a pity, considering especially the interesting geology of their Moons (particularly Miranda and Triton) and the complex atmospheres of these two giant planets.

Keep this blog live. It will be very interesting to read future posts.

Might a Discovery class mission be possible using a combination flyby and probe? Flyby bus ejects probe weeks before encounter, it enters, flyby bus relays back results before its own flyby. Then on to some well placed Kuiper belt object for another flyby?

This is grasping at straws. There badly needs to be an order of magnitude reduction in the price of these missions. Cubsesats? And yes, I know, not much solar power out there....

The other thing is timing. Another 35years before Uranus gets to its most interesting equinoctual position.

About Me

You can contact me at futureplanets1@gmail.com with any questions or comments.
I have followed planetary exploration since I opened my newspaper in 1976 and saw the first photo from the surface of Mars. The challenges of conceiving and designing planetary missions has always fascinated me. I don't have any formal tie to NASA or planetary exploration (although I use data from NASA's Earth science missions in my professional work as an ecologist).
Corrections and additions always welcome.